The invention relates to superconducting ceramic tapes, and particularly to laminated superconducting ceramic tapes.
It is known to form superconducting ceramic conductors by laminating the superconductor tape to a support structure to improve the tape's tolerance to tensile and bending stresses. However, when an article containing superconductor tape is immersed in a liquid cryogen, any contact between the cryogen and the superconducting ceramic, whether at an exposed surface of the tape or through defects in the matrix surrounding the superconducting material, allows the liquid cryogen to enter the porous ceramic structure. When the article is quickly warmed, which is standard practice in the operation of superconducting systems, the liquid cryogen trapped in the interstices of the ceramic expands quickly, creating "balloons" in the matrix and damaging the intragrain bonds in the ceramic. This results in decreases in the mechanical strength and critical current carrying capacity of the article. Typically this is true even of laminated tapes, as the cryogen seeps into the gaps between the superconducting tape and the support structure. Thermal cycling, which is a necessity for the normal operation of many superconducting articles increases this effect. To address this issue, it has been known to "pot" certain superconducting articles, such as small coils with thick layers of epoxy to minimize the likelihood of contact between the liquid and the superconducting tape, or to use other forms of cooling, such as conduction cooling when the articles cannot reasonably be protected in this manner. However, for many applications, including transmission cables, transformers, and high field magnets, the preferred cooling method requires direct thermal contact of a liquid cryogen with the superconducting conductor. In these applications, conduction cooling is typically not adequate and applying a heavy epoxy coating is typically not feasible for a number of reasons which may include, for example, packing factor and flexibility requirements as well as the need to assure good heat transfer between the article and the liquid cryogen.